This invention relates to internal combustion engine pistons and more particularly to a thermally insulated piston which restricts flow of heat from the combustion chamber through the piston body.
The efficiency of internal combustion engines, and in particular diesel engines, is improved by retaining heat from the ignited fuel in the combustion chamber of the cylinder by minimizing heat loss through the piston. This has been done in the past by insulating the piston cap by various techniques described below. A major problem, however, has been in the method of holding the ceramic cap in place under operating conditions. To date, no reliable bond between the curved surfaces of the ceramic member and the piston body has been devised.
U.S. Pat. No. 4,242,948 describes a piston which uses a metal bolt to attach a ceramic cap to the piston body. The metal bolt, however, expands during operation and tends to loosen the connection of the cap to the piston body. Any ceramic chip between the cap and the piston body will cause the cap to break when the engine cools to room temperature and the bolt shrinks. Another proposal is to shrink fit a carefully ground ceramic liner into a steel piston cap. This construction is susceptible to failure from ceramic cracks due to the irregular shear force which exceeds the mechanical property of the ceramic material.
In general, success in insulating pistons with ceramic material has been limited because of the difficulty of attaching the ceramic piece to the metal piston body without adversely stressing the ceramic piece during operation of the engine.
Application Ser. No. 495,849 provides a thermally insulated piston constructed without the use of ceramic inserts or the like. It describes the use of a honeycomb internal support insulation.
The insulated piston described in this disclosure is a more cost effective design.